The so-called light-scattering method provides the basis as prior art. This generally known technique is described in European Published Patent Application No. 358 982, British Published Patent Application No. 2 126 712 or U.S. Pat. No. 4,162,126.
For a reliable sealing function at points where shafts pass through housing walls, it is necessary also to take into account not only the sealing ring provided with an annular radial sealing lip but also the properties of the mating bearing surface on the shaft side. It usually comprises circumferentially ground shaft journal surfaces. Other possibilities of fine finishing are burnishing, roller-burnishing, external rubbing and high-precision cutting. The design engineer prescribes that the ground structure of the shaft journal should not only have specific roughness values but also be free from helical grooves. Freedom from helical grooves means that the ground structure lies exactly in the circumferential direction and there are no superimposed regular portions of the shaft.
Until now, it has largely been assumed that the so-called plunge-cut grinding method produces structures which are free from helical grooves. However, even with the unreliable so-called thread method—more details on this below—it is possible to prove that, at least with a certain combination of working parameters, even when the plunge-cut grinding method is used, helical groove structures may be produced on the workpiece surface which has been finely finished in this way.
The radial sealing lip of a sealing ring bears with a rubber-elastic sealing edge against the surface of the shaft journal with a defined radial force and over a specific axial width. As a result of the rotation of the shaft journal, the bearing region of the sealing lip is deformed to different degrees in the circumferential direction as a function of the local radial bearing pressure; smaller degrees of deformation are found near the edge and relatively severe circumferential deformations are found more in the centre region of the bearing strip. This leads to a sensitive tribological and rheological equilibrium with an oil flow which ensures the lubrication of the contact zone on the one hand, and a feedback mechanism which maintains the sealing function of the annular seal, on the other. This equilibrium must not be disrupted by a helical groove phenomenon in the microstructure of the mating bearing surface. A feeding action in one or the other direction, due to a helical groove, is to be avoided. In the case of a feeding effect, due to a helical groove, into the sealed interior of the housing, the seal would run dry, external contamination would be fed into the contact zone and the seal would wear prematurely and be broken. An outwardly directed feeding effect would prevent the seal running dry but would lead to an oil leak at the sealing point, which, for different reasons, must be more or less strictly avoided.
Apart from standarized methods according to DIN 3761, Part 2, Section 5.1.4 (see further below for more details), these properties have in the past also been monitored using the so-called thread method, but this method provided only very unreliable information, which, however, was frequently not noted at all. The thread method is a non-standardized method, which is therefore less widespread in the literature and is described, for example, incidentally, in German Patent No. 197 40 141 (not a prior publication) of the applicant and may also be referenced here once again. A method called a thread indicator is mentioned briefly, on page 103, in Volume No. 281 of the series of VDI progress reports on production engineering by G. Kersten “Optische und antastende Prüfung der Gegenlaufflache von Radial Wellendichtringen” [“Optical and contact testing of the mating bearing surface of radial shaft sealing rings”], VDI Verlag, Düsseldorf 1992. In the thread method, an oil-steeped thread, defined in terms of structure, material and strength, is wrapped somewhat more than 180° around the top of the horizontally aligned shaft journal; the ends which hang down are both attached to a small weight and the thread is thus loaded in a defined way. The shaft is then rotated slowly 20 times in one direction of rotation and then 20 times in the other direction. The axial displacement path of the thread on the surface of the journal is evaluated as a measure of the helical groove structure. The thread method supplies a clear measurement result. However, comparative measurements of the applicant using the thread method, on the one hand, and the present invention, on the other, have shown that the measurement results acquired with the thread method are in no way representative of the actual helical groove structure of the surface of the journal. The measurement results which can be obtained with the thread method do not in any way correlate with the observable tightness results or service lives of installed radial shaft sealing rings either.
It has proved to be particularly disadvantageous in the case of-the thread method that the thread itself can be affected by helical grooves, and this can lead to false, results. In addition, quantitative statements on the helical groove structure are impossible. Moreover, the thread method fails in the case of weak helical groove phenomena.
In accordance with DIN 3761, Part 2, Section 5.1.4, freedom from helical grooves, or the helical groove orientation can be established using the following method:                rotating the shaft under the microscope,        taking of fax film impressions, and        surface records transverse to the machining direction and at a plurality of points on the shaft circumference.        
According to this DIN standard, whether a helical groove orientation is disadvantageous can be established only by means of a test run with a change in direction of rotation. However, as with the thread method, quantitative statements are likewise impossible using the method described in DIN 3761, Part 2, Section 5.1.4. Moreover, the method according to this DIN standard requires an experienced observer and lasts a long time. The hardware required for carrying out the method is complicated and expensive.
A complicated method, operating according to the segmental scanning principle, for determining a helical groove structure is described in German Patent No. 197 40 141 of the applicant (not a prior publication) already mentioned. This method provides reliable data on all individual parameters of a helical groove structure, in particular the number of threads, helix angle, helical groove depth and feed cross section—of a helical groove structure. However, the known method, operating according to the segmental scanning principle for determining a helical groove structure presupposes a high outlay on apparatus and much attention to staff and, over and above this, is very time-consuming to carry out. The known method for determining helical grooves can therefore be used at most as a scientific basic or reference method, but not as a monitoring method which is close to being capable of use in production.
The parameters of interest for a helical groove structure are explained in German Patent No. 197 40 141 mentioned with the aid of graphical illustrations of the helical groove structure, for which reason this specification is also helpful in understanding the terms used in the present application. It should be mentioned at this juncture that the term “flute” used in the present application is to be understood in the sense of the term “corrugation trough of the helical groove structure” used in German Patent No. 197 40 141. The helical groove structure of interest here is caused at least indirectly by a grinding operation. This is a periodic, thread-type, multi-start fine structure, on which a stochastic, even finer ground structure is superimposed. in this case, corrugation peaks oriented largely in the circumferential direction and of approximately the same cross-sectional form are equidistantly adjacent and extend in thread-type fashion around the workpiece circumference. The characteristics of the periodic shape component of a helical groove structure are defined as follows in German Patent No. 197 40 141:                The periodic length of a helical groove structure is the spacing, measured in the axial direction, of neighbouring corrugation peaks.        The number of threads of a helical groove structure is determined, from the number of the corrugation peaks along the circumferential direction over the complete shaft circumference. Thread numbers to far above 100 have been observed on real workpieces.        The helix lead is equal to a multiple of the periodic length which corresponds to the number of threads.        The helix angle (which is, as a rule, a small angle far below 5°, mostly in the range of minutes) is yielded—in circular measure—from the ratio of the circumferential length to the lead height. By trigonometric conversion, it is at least possible to use this ratio value to specify the helix angle in angular degrees, as well.        The profile depth or helical groove depth of a helical groove is the depth between two neighbouring corrugation peaks.        The open feed cross section seen in an axially extending section, between two neighbouring corrugation peaks can be measured by planimeter in accordance with the helical groove representation obtained using the segmented scanning method, or can be determined by calculation, assuming a sinusoidal form of the corrugation peak flanks, from the profile depth and the spacing of the corrugation peaks. When determining the area, it is also possible to take account of round shapes or pointed shapes of the corrugation peaks by means of appropriately extended evaluation software.        